This seems to be the plural 'you', so shouldn't 'you guys' be accepted as well?
I don't think they used any forms like "you all" or "you guys" in this course, so just go with "you" whether it's plural or singular.
In this case anyway, önök is a very formal, high-register "you." Translating it to the regional/slangy-sounding "you guys" would be a definite error in tone.
I had "you guys" rejected in favor of "you all" here. I figured it was a question of formality.
However, they also accept "Who are y'all?" here, so I don't know what the thought process is.